I'm an experienced ESL teacher with an M.A. in Applied Linguistics. Do I need to pre-arrange a job or could I pretty much just show up in Ankara or Istanbul and just walk into something? Any help would be much appreciated.

If you want to be legally here, you need to arrange it in advance because of the work permit regulations. You must apply for the permit outside of turkey and that can take a month or two. You can probably get something pretty easily with your qualifications so just do a web seach and fire off a few CVs and see what happens. Dont trust schools that forget to tell you that you need a work permit in place before you begin working...